As discovered by Mond and Langer in 1889, nickel tetracarbonyl --Ni(CO).sub.4 -- readily decomposes into pure metallic nickel (Ni) and carbon monoxide (CO) within the temperature range of about 150-315.degree. C. By encouraging the nickel to deposit onto a substrate, objects comprised of pure nickel may be produced.
Nickel vapor deposition (NVD) is a process whereby a solid nickel shape is produced by depositing nickel onto a preform or mandrel. The mandrel is sufficiently heated to cause the nickel carbonyl process gas to thermally decompose on contact with the mandrel surface. Carbon monoxide, produced from the decomposition and residual nickel carbonyl gas is vented away. Nickel is deposited onto the mandrel to a uniform thickness, replicating exactly all surface textures and shapes of the underlying mandrel. The nickel form is ultimately stripped away from the mandrel as a finished nickel product.
The surface of the mandrel must be clean before nickel will deposit. Even small amounts of contaminants, such as oil and grease (organics) will inhibit nickel deposition.
Currently, mandrel formed nickel objects are manufactured in a reaction cylinder split into two halves or chambers. The upper chamber and lower chamber are separated by a large gasket such as silicone or other material having a central aperture.
The upper chamber contains the mandrel and receives the nickel carbonyl gas. The lower chamber supports the mandrel and acts to contain any leaked carbonyl gas and as a sump for a suitable heat transfer fluid. The heat transfer fluid, maintained at an appropriate temperature, is either sprayed on the underside or circulated through the mandrel via tubes or internal bores and galleries to heat the mandrel.
The heat transfer fluid is piped through the wall of the lower chamber and sprayed onto the underside of the mandrel or taken directly into the mandrel. The fluid drains from the mandrel into the bottom chamber. A pump draws out the fluid from the sump, causing it to pass through a heater and return to the mandrel. A modification is to take the fluid directly out of the mandrel by pipe and exit through the bottom chamber.
The mandrel is disposed within the aperture of the above referenced gasket. A mechanical seal is provided around the mandrel and the aperture by compressing the gasket between the mandrel and keeper plates bolted into the side of the mandrel. An additional mechanical seal is provided at the outside edge of the gasket by compressing the outside edge of the gasket between flanges affixed to the upper and lower chambers. End plates are welded to the end of the cylindrical chamber halves to seal the entire apparatus.
An additional seal can be provided at the mandrel/gasket interface by the deposition of a bead of liquid silicone between the gasket and the mandrel.
It is imperative that the upper and lower chambers are separated from one another and the seals have integrity.
Unfortunately, gasket material often offgasses at NVD operating temperatures and contaminates the mandrel. Consequently, the large gaskets are often subjected to a lengthy post cure (offgassing) at operating temperatures before the gasket is compatible with the NVD process. Moreover, gaskets are expensive and easily damaged during mandrel disassembly.
The mandrel/gasket seal, provided by compression with the keeper plates or by bonding does not always provide a leak tight seal. Vapors of the fluid can pass through the mandrel/gasket seal into the upper chamber and contaminate the mandrel.
A modification was made to the bottom chamber to pipe the exit fluid from the mandrel through the wall of the chamber and eliminate the fluid from the lower chamber. However, significant manhours are required to clean fluid spillage from the lower chamber each time the mandrel is disassembled.
Moreover, nickel carbonyl leaks through the mandrel/gasket seal into the lower chamber and deposits onto the underside of the mandrel and onto the ancillary mandrel tooling. Nickel oxide powder is formed in the lower chamber due to the occurring carbonyl reaction with trace oxygen in the inert gas used to purge the bottom chamber. The powder covers all of the inside surfaces and requires extensive cleanup between depositions.
A pressure differential between the top and bottom chamber must be closely monitored. A minor pressure differential causes the gasket to bulge. Any substantial tension on the mandrel/gasket seal will rupture the seal or the gasket.